Assertion (A) : The PRIMARY KEY constraint in SQL ensures that each value in the column(s) is unique and canno

Question 66 of 95

Q. Assertion (A) : The PRIMARY KEY constraint in SQL ensures that each value in the column(s) is unique and cannot be NULL. Reason (R) : Candidate keys are not eligible to become a primary key. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true and Reason (R) is not the correct explanation for Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.

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Concept understanding — Primary Key Identification

Primary Key Identification

Think about how you tell two students apart in a classroom. If you call out "Rahul," there might be three Rahul's. But if you say "Rahul Sharma, roll number 12," you've pointed to exactly one person. That unique identifier — the roll number — is doing the job of a primary key.

The Everyday Intuition

Every collection of things needs a way to refer to each item without confusion. In a library, every book has a unique accession number. In a bank, every account has an account number. In a school, every student has a unique admission number. These are not random — they are deliberately chosen so that no two items share the same identifier.

This is the core idea: a primary key is a single attribute (or a combination of attributes) that uniquely identifies each record in a table. No two rows can have the same primary key value, and no primary key can be empty (null).

The Precise Meaning

In database terms, a primary key serves two non-negotiable rules:

Uniqueness — every value in the primary key column must be different from every other value. If you have 100 students, you need 100 distinct roll numbers.
Non-null — every record must have a primary key value. You cannot have a student with "no roll number."

A primary key can be a single column, like a student ID. Or it can be a composite key — two or more columns taken together to create uniqueness. For example, in a table recording which books students have borrowed, neither "Student ID" alone nor "Book ID" alone is unique (a student borrows many books; a book is borrowed by many students over time). But the combination of "Student ID + Book ID + Date Borrowed" together can be unique.

Important

A primary key is not just any unique column. It is the official, chosen identifier for that table. A table can have many columns that are unique (like Aadhaar number, PAN number, and email), but only one of them is designated as the primary key. The others are called candidate keys — they could have been the primary key, but weren't chosen.

Why It Matters

Without a primary key, you cannot reliably update or delete a specific record. Imagine a school database where two students have the same name "Ananya Sharma." If you try to delete "Ananya Sharma," the database has no way to know which one you mean. With a primary key, you say "delete student with roll number 45" — and it's unambiguous. …

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Assertion (A) is correct—a PRIMARY KEY enforces uniqueness and disallows NULL values—but Reason (R) is false because candidate keys are precisely the keys eligible to become a primary key.

The assertion captures the essence of what a PRIMARY KEY constraint does in a relational database. When you designate a column (or a combination of columns) as the primary key of a table, the database management system enforces two iron rules: every value in that column must be unique across all rows, and no row may have a NULL value in that column. This dual guarantee—uniqueness plus mandatory presence—is what makes a primary key the definitive identifier for each record. Without it, you cannot reliably distinguish one row from another, and the entire relational model begins to crumble.

Now consider the reasoning offered. It claims that candidate keys are not eligible to become a primary key. This is precisely backwards. In database design, a candidate key is any column or set of columns that satisfies the two criteria above: it uniquely identifies each row, and it contains no NULL values. A table may have several candidate keys—employee ID, email address, and passport number might all uniquely identify a person—but you choose exactly one of these candidate keys to serve as the primary key. The others remain as alternate keys (sometimes called unique keys), still enforcing uniqueness but not bearing the special status of "primary." So candidate keys are not merely eligible to become the primary key; they are the only keys eligible for that role.

Important

A candidate key is defined as a minimal superkey—any attribute or combination of attributes that can uniquely identify a tuple and contains no NULL values. The primary key is simply the candidate key you select to be the principal identifier. …

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